This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background

Etodolac (ETD) is a non-steroidal anti-inflamatory antirheumatic drug. A survey of
the literature reveals that there is no method available for the determination of
ETD in pure form and pharmaceutical formulations by oxidation-reduction reactions.

Results

We describe three simple, sensitive and reproducible spectrophotometric assays (A-C)
for the determination of etodolac in pure form and in pharmaceutical formulations.
Methods A and B are based on the oxidation of etodolac by Fe3+ in the presence of o-phenanthroline (o-phen) or bipyridyl (bipy). The formation of the tris-complex on reaction with Fe3+-o-phen and/or Fe3+-bipy mixtures in acetate buffer solution at optimum pH was demonstrated at 510 and
520 nm with o-phen and bipy. Method C is based on the oxidation of etodolac by Fe3+ in acidic medium, and the subsequent interaction of iron(II) with ferricyanide to
form Prussian blue, with the product exhibiting an absorption maximum at 726 nm. The
concentration ranges are 0.5–8, 1.0–10 and 2–18 μg mL-1 respectively for methods A, B and C. For more accurate analysis, Ringbom optimum concentration
ranges were calculated, in addition to molar absorptivity, Sandell sensitivity, detection
and quantification limits.

Conclusion

Our methods were successfully applied to the determination of etodolac in bulk and
pharmaceutical formulations without any interference from common excipients. The relative
standard deviations were ≤ 0.76 %, with recoveries of 99.87 % – 100.21 %.

Background

Etodolac (ETD), 1,8-diethyl-1,3,4,9-tetrahydropyrano- [3,4-b]indole-1-acetic acid
[1], is a non-steroidal anti-inflamatory antirheumatic drug (Scheme 1). A survey of the literature reveals that there are very few reported methods for
the determination of ETD in biological fluids, pharmaceutical formulations and in
presence of its enantiomer. Of those studies reported, the techniques used include
chromatography, HPLC [2-5], GC [6-8], in addition to spectrofluorimetric [9] and spectrophotometric methods [9-11]. However, an extensive survey of the literature revealed that there is no method
available for the simultaneous determination of ETD in pure form and pharmaceutical
formulations by oxidation-reduction reactions.

Scheme 1

The chemical structure of Etodolac (ETD).

The aim of this study was to apply redox reactions in developing simple, accurate,
sensitive and reproducible assays to analyse ETD in pure form and pharmaceutical formulations,
by employing iron(III) with o-phenanthroline (o-phen), bipyridyl (bipy) and ferricyanide. This study describes spectrophotometric
methods that can be used in laboratories where modern and expensive equipment, such
as that required for GC or HPLC, is not available.

Results and discussion

Methods A and B

Absorption spectra

1, 10-Phenantholine and 2,2'-bipyridyl are organic bases whose chemical structures
are similar, and contain the iron(II) specific group (18). Methods A and B are based
on the formation of tris(o-phenanthroline)-iron(II) or tris(2,2'-bipyridyl)-iron(II)
following the reaction of ETD with Fe3+-o-phen or Fe3+-bipy respectively. The reaction proceeds through the reduction of Fe3+ to Fe2+, with the subsequent formation of an intense orange-red colouration attributable
to the complex (12–20). The absorption spectra of the coloured complexes under optimum
conditions were scanned in double beam mode against a reagent blank over the range
400–900 nm, and recorded according to general procedures. Characteristic λmax values were obtained at 510 and 521 nm for methods A and B, respectively (Figure 1). The experimental conditions were established by varying each parameter individually
(21) and observing the effect on the absorbance of the coloured species. All the spectral
characteristics, as well as measured or calculated factors and parameters, are summarised
in Table 1.

Effect of pH

Of the buffers investigated, acetate solution proved to be the optimal (universal,
phosphate, thiel, borate and acetate). A pH adjustment was necessary especially given
the acidic medium, because the reaction was affected by change in pH over the range
2.5–5.6. The optimum pH was 4.5 for both methods A and B; moreover, 4.0 mL of buffer
solution was sufficient for complete colour development.

Effect of reagent concentration

The addition of 1.0 mL of Fe3+-o-phen (method A) or 1.5 mL of Fe3+-bipy (method B) solutions was sufficient to obtain the maximum and reproducible absorbance
for 6.0 μg mL-1 of ETD. Smaller amounts resulted in incomplete complex formation. Increased concentration
had no effect on complex formation, although absorbance increased slightly owing to
the reagent background used.

Effects of temperature and heating time

The effects of temperature and heating time on the formation of the coloured complex
were also studied. The reaction of ETD with both reagents proceeded very slowly at
room temperature, with higher temperature employed to accelerate the reaction. Maximum
absorbance was obtained following heating on a water bath at 80°C for about 10 min
with both Fe2+-phen and Fe2+-bipyridyl coloured complexes. Further heating caused no appreciable change in the
colour. The complex obtained was highly stable for more than 12 h.

Method C

Absorption spectra

The formation of the Prussian blue (PB) complex was employed in the qualitative detection
of Fe (II). A deep blue complex was formed from the reaction between Fe(II) and hexacyanoferrate
(III). As shown in Figure 1, the chromogenic reagent blank (Fe(III) mixed with hexacyanoferrate (III) in acidic
medium) did not show strong absorption in the visible region of the spectrum. However,
after addition of the acidic hydrolysis product of ETD, the spectrum changed because
of the formation of the PB complex, which has a λmax of 726 nm.

The first step is the oxidation of Fe(II):

Fe2+ + [Fe(CN)6]3- → Fe3+ + [Fe(CN)6]4-

The second step is the formation of hexacyanoferrate (II) complex (PB):

The complex formed is highly insoluble (Ksp = 3 × 10-41) (22). Neither iron(III) nor ferricyanide solution absorb at 726 nm. Hence, the use
of measured volumes of reagent, and measurement against the corresponding reagent
blank gave a linear calibration for the drug. We herefore report the formation and
application of the Prussian blue complex in the development of a sensitive spectrophotometric
method for the determination of ETD.

The optimum conditions were established by varying parameters, such as iron(III),
ferricyanide and acid concentrations, reaction time and the order in which the reagents
were added:

Optimum iron(III) and ferricyanide concentrations

Having studied the effect of iron(III) chloride concentration on colour development,
it was observed that the absorbance increased as the volume of 0.2% iron(III) solution
increased, reaching a maximum upon the addition of 2.0 mL of the 0.2% iron(III) solution
to 6.0 μg mL-1 of ETD and 2.0 mL of 0.2% ferricyanide solution in a total volume of 10 mL. These
results indicate that maximum absorbance was obtained when the final iron(III) chloride
concentration was 0.04%. Larger volumes of iron(III) chloride (up to 4.0 mL) had no
effect on the sensitivity of the reaction. Similar observations were made when varying
volumes of 0.2% ferricyanide solution were added to fixed amounts of ETD (6.0 μg mL-1) and iron(III) chloride (2 mL; 0.2%), and diluted to 10 mL after full colour development.
The results of this study reveal that the concentrations of iron(III) and ferricyanide
reagents are not critical. However, 2.0 mL of 0.2% reagent solutions in a total volume
of 10 mL were used to ensure adequate reagent concentrations for higher drug concentrations.

Effect of nature of acid, and its concentration

The reaction product, Prussian blue, was found to flocculate within 20–30 min of colour
development. To delay the flocculation, acid was added after full colour development
and before dilution. Sulphuric acid was found to give more stable colour and reproducible
results compared to hydrochloric acid. A 1.0 mL volume of 10 M sulphuric acid in a
total volume of 10 mL was found to be adequate.

Effect of reaction time and stability of coloured species

The reaction is slow at 30 ± °2°C, but absorbance increased with time and reached
a maximum in 10 min, with colouration remaining stable for at least 6 h.

Effect of order in which reagents were added

After fixing all other parameters, a few other experiments were performed to ascertain
the influence of the order in which reagents were added. The following order: drug,
ferricyanide and iron(III), followed by sulphuric acid after full colour development,
gave maximum absorbance and stability, and for this reason the same order of addition
was followed throughout the investigation.

Method validation

Linearity

Under the optimum conditions described, Beer's law holds over the concentration ranges
0.5–8, 1–10 and 2–18 μg mL-1 respectively for methods A, B and C. The optimum concentration ranges of ETD that
can be measured accurately, as evaluated from the Ringbom plot, are 0.85–7.5, 2.0–9.0
and 3.0–16.5 μg mL-1 using methods A, B and C, respectively. We calculated the apparent molar absorptivity,
Sandell sensitivity, relative standard deviations for six replicate determinations
of 6.0 μg mL-1 and the regression equations (Table 1).

Sensitivity

The detection limit (LOD) for the proposed methods was calculated using the following
equation according to IUPAC definition (23):

LOD = 3s/k;

where s is the standard deviation of replicate determination values under the same conditions
as the sample analysis in the absence of the analyte, and k is the sensitivity, namely the slope of the calibration graph. In accordance with
the formula, the detection limits obtained for the absorbance were found to be 0.065,
0.104 and 0.228 μg mL-1 for the ETD-Fe2+-phen, Fe2+-bipyridyl and Fe2+-ferricyanide methods, respectively.

The limits of quantitation, LOQ, is defined as;

LOQ = 10 s/k

According to this equation, the LOQs were found to be 0.217, 0.347 and 0.76 μg mL-1 for the Fe2+-phen, Fe2+-bipyridyl and Fe2+-ferricyanide methods, respectively.

Accuracy and precision

In order to determine the accuracy and precision of the proposed methods, solutions
containing four different concentrations of ETD were prepared and analysed in six
replicates. The relative standard deviation as precision and percentage relative error
(Er %) as accuracy of the suggested methods were calculated at 95% confidence levels,
and can be considered satisfactory. Precision was carried out by six determinations
at four different concentrations in these spectrophotometric methods. The percentage
relative error was calculated according the following equation:

Er % = [(found – added)/added] × 100

The inter- and intra-day precision and accuracy results are shown in (Table 2). The analytical results for accuracy and precision show that the methods proposed
have good repeatability and reproducibility.

Table 2. The intra-day and inter-day precision and accuracy data for ETD obtained by the proposed
methods (A – C).

Effects of interference

The interference criterion was an error of not more than ± 3.0 % in the absorbance.
To test the efficiency and selectivity of the proposed analytical methods (A-C) to
pharmaceutical formulations, we carried out a systematic study of additives and excipients
(e.g. lactose, glucose, dextrose, talc, calcium hydrogen phosphate, magnesium stearate
and starch) that usually present in dosage forms. Experiments showed that there was
no interference from additives or excipients for methods A-C (Table 3).

The main degradation products of etodolac are identified as 7-ethyl-2(1-methylene
propyl)-1H-indole-3-ethanol, 1,8 diethyl-1-methyl-1,3,4,9-tetrahydropyrano- [3,4-b]indole
and 7-ethyl tryptophol [24]. Consequently, there was no interference of the degradation products in the determination
of the ETD in proposed methods.

Analytical applications

Because our three methods were successfully applied to the determination of ETD in
its pharmaceutical formulations, they could therefore be used easily for the routine
analysis of pure ETD and its dosage forms. Moreover, to check the methods' validity,
dosage forms [Napilac capsules (200 mg ETD per capsule) and Etodine capsules (300
mg ETD per capsule)] were tested for possible interference with the standard addition
method (Table 4). The methods' performance was assessed using the t-test (for accuracy) and a variance
ratio F-value (for precision) compared with the reference method [9] (for 95% confidence level with five degrees of freedom (25)). The results showed
that the t- and F-values were less than the critical value, indicating that there
was no significant difference between the proposed and reference method for ETD (Table
5). Because the proposed methods were more reproducible and had higher recoveries than
the reference method, they can be recommended for adoption in routine analysis in
the majority of drug quality control laboratories.

Table 5. Determination of ETD in pharmaceutical preparations using the proposed methods.

Conclusion

The methods proposed are simpler, less time consuming and more sensitive than those
previously published. All the proposed methods were more advantageous than other reported
visible spectrophotometric [9-11] methods with respect to sensitivity, simplicity, reproducibility, precision, accuracy
and stability of the coloured species for ≥ 12 h. The proposed methods are suitable
for the determination of ETD in pure form and pharmaceutical formulations without
interference from excipients such as starch and glucose, suggesting potential applications
in bulk drug analysis.

Experimental

Apparatus

All absorption spectra were recorded using a Kontron 930 (UV-Visible) spectrophotometer
(German) with a scanning speed of 200 nm/min and a band width of 2.0 nm, equipped
with 10 mm matched quartz cells. A Hanna pH meter (USA) was used for checking the
pH of buffer solutions.

Materials and reagents

All chemicals and materials were of analytical grade, and all solutions were freshly
prepared in bidistilled water.

Pure samples

Etodolac (ETD) pure grade was supplied by Pharco, Egypt. The purity was found to be
100.35 ± 0.64 % according to the Pharco method [26] in which the absorbance of 0.002% w/v etodolac solution in 0.1 N sodium hydroxide
was measured at 276 nm.

Standard Stock solutions

Stock solutions of ETD were prepared by dissolving 100 mg of pure drug in methanol,
followed by dilution to 100 mL with the same solvent to obtain 1 mg mL-1standard solutions. Working solutions were prepared by an appropriate dilution of
the stock standard solution.

Recommended analytical procedure

Methods A and B

100 μg mL-1 aliquots of the standard solutions (A: 0.05–0.8 and B: 0.1–1.0 mL) were transferred
to a series of 10 mL calibrated flasks. To these were added 1.0 mL of Fe3+-o-phen (method A) or 1.5 mL of Fe3+-bipy (method B) reagent solutions and 4.0 mL of acetate buffer (pH 4.5), before heating
on a water bath at 80°C for 10 min. The mixture was cooled to room temperature (25
± 1°C) and the volume made up to the mark with bidistilled water. The coloured complexes
formed were measured at 510 and 521 nm against a reagent blank treated similarly according
to methods A and B.

Method C

Into a series of 10 mL calibrated flasks, aliquots (0.2–1.8 mL) of 100 μg mL-1 standard solutions were transferred using a micro burette and the total volume adjusted
to 3 mL by adding bidistilled water. Then, 2 mL each of FeCl3 (0.2 %) and K3Fe(CN)6 (0.2 %) were added to each flask, mixed well and left to stand for 10 min. Finally,
1 mL of 10 M H2SO4 was added to each flask, diluted to the mark with bidistilled water and mixed well.
The absorbance of the resulting solution was measured at 726 nm for ETD against a
reagent blank prepared similarly. A calibration graph was constructed by plotting
the absorbance against the drug concentration or calculated regression equation.

Analysis of pharmaceutical formulations

Ten tablets were accurately weighed and powdered. An accurately weighed quantity equivalent
to 20 mg ETD was dissolved in 20 mL of methanol and transferred to a 100 mL calibrated
flask. The contents of the flask were shaken for 10 min, and then made up to the mark
with methanol. The general procedure was then followed for concentration ranges already
mentioned for methods A, B and C.